An electric motor is a device which converts electrical energy to kinetic energy. An electric motor may be classified as DC motor or AC motor, inner-rotor type or outer-rotor type based on positional relationship of a stator and a rotor, and winding-field type or permanent-magnet type. Either classification includes an electric motor which utilizes so-called revolving-field, in which a stator revolves field direction to influence a rotor to rotate.
In a revolving-field motor constructed with a stator including a cylindrical coil and a rotor sandwiching the cylindrical coil and forming air gap, it is well-known that heat is generated by resistance (copper loss) of the cylindrical coil, eddy current at an inner yoke and an outer yoke of a conductor forming the cylindrical coil and the air gap, and hysteresis of an iron core. It is also well-known that copper loss or hysteresis which converts magnetic energy to thermal energy is an inevitable technical problem.
To address influence to output and/or efficiency of an electric motor induced by the above technical problem and heat-induced deterioration of coercive force of permanent magnets equipped on an outer surface of an inner yoke and/or on an inner surface of an outer yoke which configure a rotor, it has been attempted to cool coiled surface of windings, but it does not yet lead to fundamental solution. The inventors of the present invention have challenged the technical problem and developed a coreless rotating electrical machine comprising a stator including a cylindrical coil and its cooling method.
Patent Document 1 (Japanese Laid-Open Patent Publication 2012-016218A) describes a wheel-in motor with a coreless type coil. More specifically, this electric motor is a motor in which a cylindrical-shaped outer yoke integrated with a wheel and a cylindrical-shaped inner yoke forming air gap between the outer yoke configure a rotor rotatably mounted to a fixed shaft, a cylindrical-shaped coil arranged in the air gap configures a stator coupled with the fixed shaft, and permanent magnets equipped in an inner surface of the outer yoke configuring the rotor are configured to be opposingly arranged on an outer surface of a coil configuring a stator.
Patent Document 2 (Japanese Laid-Open Patent Publication 2012-030786A) describes a wheel-in motor with a coreless type coil having a similar configuration of the rotor and the stator described in Patent Document 1. The electric motor described in Patent Document 2 further includes a braking means fixed to an inner yoke in a space formed in an inner surface of the inner yoke of a rotor.
Patent Document 1 does not describe for a cooling means against heat generated when operating an electric motor, whereas Patent Document 2 describes as that the electric motor thereof includes a cooling means for a space in the motor which is formed with an inner surface of the inner yoke and an inner surface of the wheel by opening an end face of the wheel fixed to the outer yoke to the stator and using the end face as a vent hole to allow for communicating an interior of the motor with ambient air. The vent hole allows for communicating the space formed on the inner surface of the cylindrical inner yoke with ambient air and is considered as a cooling means for mitigating frictional heat caused by braking, but it is not intended to directly cool air gap formed with a coil, an inner yoke and an outer yoke, which is a later-described feature of the present invention.
Patent Document 3 (Japanese Patent No. 2,657,192B) discloses a linear DC brushless motor which is a linear motor configured with a slider arranged with field magnets which relatively slide with respect to a fixed armature, and is not a revolving field motor which is an object of the present invention.
The linear motor has an air supply channel drilled in the fixed armature and is “configured to directly blow air from the air supply channel to the armature coil to cool the armature coil as well as to cool a stator yoke itself with respect to a magnet yoke”. The fixed armature is configured with a stator yoke in which a plurality of rectangular air-core coil, formed by winding a lead wire for multiple turns, is bonded on a printed wiring board in moving direction of a slider, but the coil formed by winding a lead wire for multiple turns may not be thoroughly cooled even by directly blowing air to the armature.
Patent Document 4 (Japanese Laid-Open Patent Publication 2006-246678A) describes an outer-rotor type wheel-in motor. There is described a cooling means for a coil formed by winding a lead wire for multiple turns which is mounted on 6 salient poles on a stator side in a SR motor configured with 6 salient poles on a stator side and 4 salient poles on a rotor side provided on a hollow shaft.
In the cooling means, an inflow channel and an exhaust channel are provided via a bulkhead, air is supplied to a coil through the inflow channel, and air after contacting the coil is discharged to outside of a stator through the exhaust channel. Patent Document 4 thereby presents a wheel-in motor in which a channel is formed for exhausting air after cooling a heated coil. However, this cooling means, similarly as that of Patent Document 3, may not allow for thoroughly cooling the lead wire which is wound for multiple turns even by blowing air directly to the coil, because the air flows only on an exposed surface of the lead wire which is wound for multiple turns.
Patent Document 5 (Japanese Patent No. 3,494,056B) describes an outer rotor type magnetic generator comprising a stator with a coil wound around an annular stator core and a rotor consisting of an outer yoke with permanent magnets being supported on an inner surface of a cylindrical part which covers an outer periphery of the stator, wherein the rotor is fixed to a rotating shaft which the stator is rotatably coupled thereto.
In the electric motor, a vent hole is provided on a plate which supports a stator rotatably coupled to the rotating shaft, the vent hole on the plate is communicated with a vent hole provided on a bottom of a rotor for cooling a coil wound around a stator core and an inner surface of the permanent magnets, the rotor is rotated so that air is intaken from the vent hole on the plate and is discharged from the vent hole of the rotor, the discharged air is further blown to the cylindrical part of the rotor for cooling the permanent magnets through the cylindrical part.
Patent Document 6 (Japanese Laid-Open Utility Model Publication H5-022133A) describes a cooling means for forced cooling of interior of an outer rotor type wheel-in motor for an electric vehicle. The electric motor includes a cooling means which intakes ambient air to interior of a stator from a vent hole on a hollow shaft through a filter by a cooling fan which is in communication with the hollow shaft, and exhausts the air after contacting a coil of the stator and the inner surface of the rotor from an exhaust hole on a partitioning plate.
Patent Document 7 (U.S. Pat. No. 6,873,085) describes a cylindrical coil formed in a cylindrical shape to be mounted in a coreless type coil motor including a fixed armature and in a rotating armature. However, the electric motor does not have a cooling means for the cylindrical coil and air gap formed with a cylindrical coil, an outer yoke and an inner yoke.
Patent Document 8 (Japanese Patent No. 3,704,044B) describes a coreless armature for a DC motor, which includes a cylindrical coil processed with patterns which generate a series of substantially parallel conductors being mutually separated by polyamide which is an insulating material.